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Filling out a date range with values based on most recent

Posted on 2014-12-01
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Last Modified: 2014-12-01
I have a table like this:
tblChanges
Date     Value
1/1           3
5/1           5
7/1           8

And another like this:
tblDates
Date
1/1
2/1
3/1
4/1
5/1
6/1
7/1
8/1

I want to combine these tables into this
Date  Value
1/1    3
2/1    3
3/1    3
4/1    3
5/1    5
6/1    5
7/1    8
8/1    8

So in other words, it's like a left join from tblDates on tblValues where nulls yield that last (i.e. most recent) change value.  Restated, I want to know what change value is in effect for each date in the list.  If nothing matches, take the most recent.

This could be done with Domain functions (like DLast), but that's very cumbersome.  Is there a better way?
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Question by:shacho
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5 Comments
 
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Expert Comment

by:Phillip Burton
ID: 40473287
It may be cumbersome, but it's probably the best way.

If this was SQL Server, then you could use the LAG function, but that's not present in Access.

Outside of DLast, you would have to do some JOINs with non-equal comparisons, and if you have a big table, that would get slow - so Domain functions are probably the best for Access.
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Accepted Solution

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Phillip Burton earned 500 total points
ID: 40473343
Here's the SQL for the query with non-equal comparisons. It might not be as slow as I was fearing:

SELECT qryRelevantDate.Date, tblChanges.Value
FROM (SELECT tblDates.Date, Max(tblChanges.Date) AS MapDate
FROM tblDates INNER JOIN tblChanges ON tblDates.Date >= tblChanges.Date
GROUP BY tblDates.Date) as qryRelevantDate INNER JOIN tblChanges ON qryRelevantDate.MapDate = tblChanges.Date;

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LVL 24

Expert Comment

by:Phillip Burton
ID: 40473344
This is the output:

Date	Value
01/01/2014	3
02/01/2014	3
03/01/2014	3
04/01/2014	3
05/01/2014	5
06/01/2014	5
07/01/2014	8
08/01/2014	8

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LVL 49

Expert Comment

by:Gustav Brock
ID: 40473443
Why not pick the values directly:

Select
    tblDates.Date,
    (Select Max(tblChanges.Value)
    From tblChanges
    Where tblChanges.Date <= tblDates.Date) As [Value]
From
    tblDates

/gustav
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Author Comment

by:shacho
ID: 40473920
Yep, that does the trick, Phillip.
Thanks all for your comments!

Cheers,
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